This invention relates to microelectromechanical structures, and more particularly, this invention relates to microelectromechanical structures using ceramic substrates.
Thick film materials have been used for packaging applications in the microelectronics industry. Their acceptance in the industry, however, has not been widespread because thin film technology using silicon photolithographic processes has been better adapted for high performance requirements, such as microwave and other radio frequency applications. The thick film substrates also have poor geometrical resolution and a very high dielectric constant. Via formation has not been adequate, and typically greater than 200 microns.
In recent microelectronic packaging designs, there has been a greater need for high performance materials and processes that allow finer geometries and circuit layout without sacrificing performance. As a result, for critical applications as in the fabrication of microstrip transmission lines and similar structures, thin film silicon and other related thin film technology has been used instead of thick film processing technology.
Three major approaches for fabricating microstrip transmission lines and similar structures have been thin film, thick film and polymer technology, i.e., soft board. These technologies have some drawbacks with regard to current and future applications. Thin film technology has become established for use with microwave circuit and component technology, but it is expensive because of its typical use of sputter coating, which is also wasteful in material. Additionally, an adhesion layer is required between a sputtered line and a substrate to create an insulator at the interface with the substrate, where current flows in the microstrip line. Because a thin film material follows the surface finish of a substrate, losses increase, which are significant with increasing frequency. Thus, an expensive 99.6% polished alumina has been used for thin film applications, and increases even more the cost of a thin film microwave or radio frequency circuit. Even then, this type of circuit will suffer losses due to the presence of the adhesion layer.
Polymer technology is used increasingly for microwave circuits because it has a lower cost. These lower cost polymer materials, however, are high in dielectric loss and have limited component geometries and some environmental limitations. Some of the more advanced materials overcome these drawbacks, but are even more expensive. Even though geometrical constraints can be overcome by the use of a build-up technology, the costs increase.
Thick film technology, such as the well known green tape technology and other similar materials technology, is little used at the present for microwave circuit applications and similar radio frequency applications, even though it is an inexpensive medium. Thick film circuit technology can resolve many microstrip lines, but it occurs with poor edge definition and a relatively rough conductive surface. For some components, such as an edge coupled filter, it does not produce the fine gaps required for the design. Also, the dielectric materials generally are high in dielectric constant (seven or greater) and have a dielectric loss of 0.01, i.e., about 1%. The low cost of the thick film materials, however, would make them advantageous for use in microelectromechanical structures used with radio frequency applications, if a means could be found to overcome the drawbacks as indicated above.
The present invention is advantageous and now allows the use of thick film materials for use in microelectromechanical structures used in radio frequency and similar microwave applications. A microelectromechanical structure of the present invention includes a ceramic substrate formed preferably as a low temperature co-fired ceramic substrate. A low loss photodefinable dielectric planarizing layer is formed over the low temperature co-fired ceramic substrate and could be a sacrificial layer. Alternately, a sacrificial layer could be formed over the planarizing layer and could be a material that could be etched. A photodefined conductor is printed over the low loss dielectric planarizing layer or sacrificial layer and formed together with the sacrificial layer into a structural circuit component preferably for use with radio frequency applications. In one aspect of the present invention, the photodefined conductor is formed as a switch. A biasing actuator is formed and a deflectable member is formed over the biasing actuator and movable into open and closed circuit positions. This deflectable member can be formed as a cantilever beam or formed as a suspended beam, depending on the application.
In still another aspect of the present invention, the photodefined conductor can include an input signal line and output signal line that transfers current when the switch is in the closed, circuit position. The photodefined conductor comprises a thick film conductor and the photodefinable dielectric planarizing layer is preferably formed from a borosilicate based, thick film dielectric material. It typically has a dielectric constant of about 3.8 to about 4.2, and on the average is about 3.9. It also has a loss factor of about 0.01%, i.e., 0.0001. The ceramic substrate is preferably formed from ceramic sheets that are stacked and co-fired together. These sheets can have printed circuit connections and vias extending along various surfaces of the layers for interconnection to various circuit components, as required by those skilled in the art.
A radio frequency switch circuit is also disclosed and includes a radio frequency input, control input, radio frequency output and the microelectromechanical switch of the present invention connected to the radio frequency input and output, and the control input. The control input can include a transistor. The radio frequency input can include a coil and capacitor combination as required, and a positive terminal connection. The control input is typically connected to base and the radio frequency output includes a capacitor as required in some instances.
A method of forming a microelectromechanical structure is also disclosed and comprises the step of forming a ceramic substrate and forming a low loss photodefinable dielectric planarizing layer over a surface of the low temperature co-fired ceramic substrate. A structural circuit component is formed on the photodefinable dielectric planarizing layer from a sacrificial layer and a photodefined conductor preferably for use in radio frequency applications. The ceramic substrate can be formed from ceramic sheets that are stacked and co-fired together. The photodefined conductor is formed with the sacrificial layer as a switch having a biasing actuator and deflectable member formed over the biasing actuator and moveable into open and closed circuit positions.